The present invention relates to a signal driver circuit for a liquid crystal display (LCD), and, more particularly, to a dual mode thin film transistor liquid crystal display (TFT-LCD) source driver circuit having low power consumption.
Due to the increased demands for data, handheld communication and portable electronics equipment, such as radios, cellular and cordless telephones, pagers, personal digital assistants (PDAs) and the like, must display greater amounts of information. Equipment must provide displays which feature visual messages that include graphics and printed information as well as a means to access and manipulate such messages. Accordingly, equipment must provide displays that accommodate text and icon information, as well as graphic and video data. Most circuitry used to implement these and other features expend relatively large amounts of power. As a result, power consumption is a major concern for many handheld communication and portable electronics manufacturers.
Conventional liquid crystal displays (LCDs) provide these features using two sheets of polarizing material having a liquid crystal solution between the two, such that when an electric current passes through the liquid, the crystals align to block or pass light. Each crystal, therefore, acts like a switch, either allowing light to pass or blocking light.
Source driver circuits are commonly employed with liquid crystal displays. The driver circuit typically accepts digital video data as an input and provides an analog voltage output to each particular LCD pixel column. Generally, each column in the LCD must be uniquely addressed by a signal or column driver and given the proper analog voltage in order to achieve the desired transmissivity (i.e., the desired shade of gray or color). Moreover, it is desirable that the output voltage range of a driver circuit be wide to allow for a high pixel contrast ratio.
For color LCDs, each pixel is composed of 3 sub-pixel elements representing the primary colors of red, green and blue. For example, a color VGA panel having a resolution of 640 columnsxc3x97480 rows of uniquely addressable pixels will have 3xc3x97640 columns, or 1920 columns. Typically, the signal driver circuit has one driver output for each column. Thus, controlling an LCD panel requires a large number of driver outputs that consume considerable circuit area and power. Since this large number of circuitry size impacts power consumption, it is desirable to provide stages of operation in which the operation of each driver circuit is suspended.
Conventionally, there are two modes of operation: standby and gray scale mode. There are two types of standby mode where operation of parts of the source driver is suspended. The first type of standby mode powers down all of the internal circuitry with the exception of some input signal detection circuitry. Given this mode, however, the driver provides no output signal. The second type of standby mode powers down some of the internal circuitry during normal operation of the circuit to save power, not altering the overall system behavior. In gray scale mode, a full color display is present at the LCD providing up to 262144 colors. Since it is common for the communications equipment to remain in standby mode or text mode, where only text or icon display on the panel, it is not necessary to display full color display quality.
An approach to lower power consumption may include the use of a color super-twisted nematic liquid crystal display (STN-LCD). Although this implementation provides the greatest benefit, there exists slow display response time. In addition, using STN-LCD makes it difficult to generate high resolution colors. Both of these problems contribute to the complexity of displaying real time video or graphic information.
Another approach to lower power consumption may include the use of color LCD displays using the thin film transistor (TFT) technology which produce color images that are as sharp as traditional CRT displays. The TFT-LCD is a type of LCD flat-panel display screen, in which each pixel is controlled by one to four transistors. Conventional, TFT-LCDs can provide higher display response time and high resolution colors, but the power consumption is ten times that of STN-LCD. As a further limitation to the TFT-LCD implementation, the light transmission curve shown in FIG. 4 illustrates that the conventional TFT-LCD source driver is useful during a limited range of the voltages.
Thus, there exists a need for a dual mode TFT-LCD source driver circuit having low power consumption that is operable in response to a large range of voltages having at least one type of standby mode where operation of a portion of the driver circuit is suspended to lower power consumption such that the LCD is still capable of providing text, icon, graphic and video information on the display without using the full scale of colors available in the gray scale mode.
To address the above-discussed deficiencies of the dual mode thin film transistor liquid crystal display source driver circuit, the present invention teaches dual mode thin film transistor liquid crystal display source driver circuit having low power consumption. A first embodiment of the source driver circuit including a data inputs which connect to sample registers. An N-bit shift register containing N is the uniquely addressable channels couples to the sample registers. The input data is indicative of an image to be displayed on the LCD. Hold registers couple to the sample registers to store the sampled data. The hold register receives a transfer signal to determine when the data from the sample register should be transferred to the hold register. A resister string can provide up to 64 voltage levels for example which couple to a set of decoder cells that are programmable to decode the input data to select respective output voltage levels. Output cells couple between the hold register a set of driver outputs. A set of switches connect each respective decoder cell to the driver outputs. Both the set of switches and output cells couple to receive a mode signal, such that two modes of operation exists. In the first mode, when each switch is closed, the output cells are bypassed and, in the second mode, when each switch is open, the decoder cells are bypassed. This provides for a gray scale mode having full color display resolution and a standby mode that decreases the amount of power dissipated yet presents voltage output for the LCD to provide text, icon, graphic and video data.
In an alternative embodiment, latch circuits are employed which vary the level of voltage output during the standby mode. Thus, video displays may be programmed to have a specified resolution while still conserving power. The latch circuits may couple between the sample registers and the hold registers or between the hold registers and the output cells.
Advantages of this design include but are not limited to dual mode thin film transistor liquid crystal display source driver circuit having low power consumption.